If a ball is thrown horizontally with an initial velocity of 30 m/s. hits field 4 seconds later. What is its speed the instant before it hits the field?

Assuming no friction, horizontal component v_x=30m/s will not change. Vertical component increases as
a free fall, thus v_y=gt, thus after 4 seconds you have v_y=9.81 m/s^2 \times 4 s .
The total speed can be obtained from
v=\sqrt{v_x^2+v_y^2}.